Scanning probe microscopes enable a class of imaging techniques in which a tip that interacts locally with a sample is scanned over the surface of the sample to generate a three-dimensional image representing the properties of the surface. For example, in contact mode atomic force microscopy, as the tip is moved over the surface of the sample, the arm deflects in response to the changes in topology of the surface. The vertical position of the cantilever arm relative to the sample is adjusted to maintain the arm in a predetermined state. The vertical position as a function of position on the sample can then be used to provide an image of the surface.
Images are typically acquired in one of two modes. In the contact or constant force mode, the tip is brought into contact with the sample and the tip moves up and down as the tip is moved over the surface. The deflection of the arm is a direct measure of force and topographical variations. A feedback controller measures the deflection and adjusts the height of the probe tip so as to maintain constant force between the cantilevered probe and the surface, i.e., the arm is maintained at a fixed deflection. This mode of operation can subject the sample being imaged to excessive shear forces that can alter or damage the sample. This limitation is particularly acute when biological samples or macromolecules are being imaged. Accordingly, some form of non-contact mode of scanning is often preferred for these applications.
In the AC, or non-contact mode, the tip and arm are oscillated at a frequency near the resonant frequency of the arm. The height of the tip can be controlled such that the tip avoids contact with the sample surface, sampling short-range tip/sample forces. Alterations in the short range forces between the tip and the sample result in changes in the oscillation frequency of the tip. Alternatively, the tip can be allowed to make light intermittent contact with the sample only at the bottom of the oscillation cycle. Contact between the probe tip and the sample results in an alteration of the amplitude, phase and/or frequency of the oscillation. The controller adjusts the height of the probe over the sample such that the oscillation amplitude, phase and/or frequency is kept at a predetermined constant value. Since the tip is not in constant contact with the sample, the shear forces applied to the sample are significantly less than in the mode in which the tip is in constant contact. For soft samples, this mode reduces the damage that the tip can inflict on the sample and also provides a more accurate image of the surface in its non-disturbed configuration.
The image is constructed one point at a time and limited by the rate at which the tip can be moved relative to the sample, as well as the time required for the servo loop to reposition the tip vertically, to maintain the distance between the surface and the tip in DC mode, or to keep the oscillation parameter of interest at the predetermined constant value in AC mode. Also, in AC mode, the time needed to detect a change in one or more of the oscillatory parameters of the arm is many periods of the oscillation. Hence, each time the position of the arm is moved relative to the sample, the servo system must wait for a period of time that is long compared to the oscillation period to determine the new amplitude, phase, or frequency of the oscillation and then alter the distance between the arm and the surface to return the oscillatory parameter of interest to the desired value. As a result, the time needed to provide an image can be excessive.